The present invention relates to a structure of the lead terminal of an electronic device such as a semiconductor device or the like, and in particular, to a structure of the lead terminal of an electronic device of a surface mounting type.
Conventionally, most of electronic devices have been connected to a printed wiring board by inserting the lead terminal thereof into a through hole formed in the printed wiring board and soldering it. However, in recent years, as electric machines and apparatuses have been made compact in size and thin in thickness, a so-called surface mounted type of electronic device has been widely employed, which can be connected to the printed wiring board by bringing the lead terminal thereof into contact with a land provided on the printed wiring board and soldering it.
FIG. 5 shows an external appearance of a typical electronic device of a surface mounting type. In FIG. 5, reference numeral 1 designates a package main body in which a semiconductor element or the like is hermetically sealed. The package main body can be produced, for example, by molding an epoxy resin. A lead terminal 2 is extended from the package main body 1 and the lead terminal has a surface which is plated by soldering or by similar methods. Each of the lead terminals 2 is formed in such a manner that it is bent in a substantially L-shaped configuration, the leading end portion 2a thereof can be brought into contact with a land provided on a printed wiring board and then can be connected by soldering onto the land.
FIG. 6 is a sectional view to show a state in which the above-mentioned electronic device is soldered to a printed wiring board. In this figure, reference numeral 3 designates a printed wiring board, 4 designates a land provided on the printed wiring board 3, and 5 designates a solder which is used to connect the leading end portion 2a of the lead terminal 2 with the land 4. A method of soldering the electronic device of a surface mounting type includes a so-called re-flow method in which the lead terminal 2 and the land 4 are previously coated with a solder film by solder plating or the like and then the printed wiring board 3 with the electronic device mounted thereon is put into a high temperature atmosphere or a laser beam is applied to the soldered portion to thereby solder the lead terminal 2 to the land 4, a method which uses a solder paste to connect the lead terminal 2 and the land 4 with each other, a method in which, with the electronic device being mounted on and fixed to the printed wiring board 3 with an adhesive, the printed wiring board 3 is immersed into a solder tank to thereby achieve such soldering connection, and other similar methods.
However, the above-described conventional electronic device to be soldered by the above-mentioned methods has some problems to be solved as follows:
That is, since the leading end portion 2a of the lead terminal 2 of the conventional electronic device is formed in a flat-plate shape, a gas from flux or the like produced when the printed wiring board 3 and the land 4 are soldered together, is easy to enter between the leading end portion 2a of the lead terminal 2 and the land 4, which makes it difficult for the solder to enter into the gas existing portion to thereby generate poor soldering.
Also, since the area of the leading portion 2a of the lead terminal 2 of the electronic device of this type is too small to have an adequate contact area with the solder, the soldered portion may be peeled off by a stress generated when the printed wiring board 3 is warped.